"It's ok for prosecutors and police to use every means legally at their disposal to try and prove someone guilty, as long as that person has an independent advocate with sufficient authority to protect the accused and challenge the prosecution case."
I don't necessarily agree with that. The defense should be using every means available, the prosecution should restrict themselves to things that get at the truth. As extreme examples, presenting fabricated evidence or withholding exculpatory evidence would clearly be unacceptable even if laws were changed to allow it. I am not remotely confident that laws proscribe everything they should proscribe.